Heretofore, a cordierite type ceramic filter or silicon carbide type ceramic filter has been proposed as a filter to remove dust, etc. contained in a high temperature exhaust gas. However, the cordierite type ceramic filter is not necessarily adequate from the viewpoint of heat resistance and corrosion resistance although it is excellent in thermal shock resistance, and the silicon carbide type ceramic filter is not necessarily adequate with respect to the thermal shock resistance, although it is excellent in heat resistance and corrosion resistance.
Particularly when the ceramic filter is one intended for arresting diesel particulates (hereinafter referred to simply as particulates) discharged from a diesel engine (hereinafter referred to simply as an engine), it has been likely with the above-mentioned cordierite type filter or the silicon carbide type filter that the particulates arrested by the filter will locally burn to cause a melting loss, thus presenting a fatal damage to the ceramic filter. Further, the particulates contain a sulfur content and a phosphor content, whereby acid resistance is required, but the cordierite type filter used to be not necessarily adequate with respect to the acid resistance.
On the other hand, silicon nitride has excellent characteristics with respect to heat resistance, thermal shock resistance, corrosion resistance, acid resistance, mechanical strength, etc., and is expected to be useful as a filter for dust arresting or dust removing in a high temperature or corrosive environment. Especially, silicon nitride is excellent in heat resistance, thermal shock resistance, acid resistance and mechanical strength, and is thus considered to be a material suitable for a filter for particulates.
As a method for producing such a silicon nitride filter, several have been proposed.
For example, JP-A-6-256069 proposes a method of firing a green body comprising silicon nitride particles, clay and an oxide. Further, JP-A-7-187845, JP-A-8-59364 and JP-A-6-24859 propose methods of using as starting materials a mixture comprising silicon nitride particles and an organic silicon compound, a mixture comprising silicon nitride particles and a polysilazane and a mixture comprising silicon nitride particles and a synthetic resin foam, respectively. However, such methods of using silicon nitride particles as starting materials have had a problem that as compared with a method of using metal silicon particles as the starting material and converting it to silicon nitride by direct nitriding, pores with pore diameters of at most 1 μm are little, whereby the Young's modulus is high, the thermal shock resistance tends to be poor, the production cost tends to be problematic since the silicon nitride particles are relatively expensive.
On the other hand, as a method of employing metal silicon particles, JP-A-1-188479 proposes a production method to obtain a porous product having a nitriding ratio of the metal silicon particles of at most 50%, by using as a starting material a mixture comprising metal silicon particles and silicon nitride particles. However, in this method, the nitriding ratio of the metal silicon particles is at most 50%, whereby there will be a substantial amount of silicon metal remaining in the silicon nitride sintered body in the form of metal silicon without being nitrided, whereby there will be a problem such that excellent heat resistance or corrosion resistance of silicon nitride will be impaired.
Further, by the method of using metal silicon particles, sintering of the formed silicon nitride particles is usually not sufficient, whereby the mechanical strength of the porous body thereby obtained, tends to be inadequate.